1. Field of the Invention
This invention relates to the field of heat exchangers. More specifically, the invention comprises a falling film liquid-to-liquid plate type heat exchanger featuring an enhanced surface on the primary fluid side.
2. Description of the Related Art
Heating, ventilation, and cooling (“HVAC”) systems employ at least two heat exchangers in a circulating loop of refrigerant. One heat exchanger is used to reject heat. This device is typically known as a condenser. A second heat exchanger is used to absorb heat. This device is typically known as an evaporator. Some HVAC systems have reversing valves which allow the circulating loop of refrigerant to selectively operate in either direction. When the flow is reversed, the evaporator becomes the condenser and the condenser becomes the evaporator.
Most readers will be familiar with residential HVAC systems. Such systems employ liquid-to-air heat exchangers. As an example, a residential heat pump uses one heat exchanger located outside the dwelling and another typically located inside the dwelling. When the heat pump is operating in the air conditioning mode, the external heat exchanger operates as a condenser. Hot compressed refrigerant gas is piped from the compressor to the external heat exchanger, where it loses heat and condenses to a liquid. The exchanger itself is a serpentine conduit passing through a large array of connected cooling “fins.” These fins greatly increase the effective surface area of the conduit, so that heat is transferred from the hot gas to the surrounding air. A fan is often used to force air over the fins.
The internal heat exchanger operates as an evaporator. Liquid refrigerant is passed through an expansion valve which lowers the pressure, thereby lowering the boiling temperature of the liquid, and the liquid is then boiled off inside the evaporator as it absorbs heat from the environment surrounding the evaporator (as it flows through the evaporator). The evaporator is typically another serpentine conduit connected to an array of fins. Air is forced over this exchanger. Heat is transferred from the air to the gaseous refrigerant, which cools the air. The cooled air is then circulated throughout the dwelling.
In a residential HVAC system, the refrigerant is said to be the “primary fluid” and the circulating air is said to be the “secondary fluid.” This arrangement works reasonably well for structures of moderate size. However, when considering larger structures, the use of air as a secondary fluid becomes inefficient. An HVAC system for a skyscraper, for example, may need to circulate the secondary fluid over 1,000 feet. The frictional losses using air over such a distance are substantial. Further, the heat carrying capacity of air is limited by its relatively low density. For these reasons, commercial HVAC systems typically use water as the secondary fluid.
In a commercial HVAC system used in the air conditioning mode, the evaporator exchanges heat between the circulating refrigerant (primary fluid) and circulating water (secondary fluid). This type of heat exchanger is often referred to as a “chiller.” The chilled water is then pumped throughout the building. At various locations the chilled water is passed through a water-to-air heat exchanger where it absorbs heat and cools the air. The cooled air is then circulated in a particular region to cool the building. Such a system has a large volume of circulating cooled water which can be used selectively to cool portions of the building where it is needed.
Heat exchangers typically used for the chiller (evaporator) function are the shell and tube type, the direct expansion brazed plate type, the flooded shell and tube type, the flooded brazed plate type, the falling film type, and the flooded shell and plate type. Heat exchangers typically used for the condenser function include the shell and tube type, the fin and tube type, the brazed plate type, the evaporative assisted type, and the direct evaporation cooled type.
In larger more efficient systems the flooded shell and tube evaporators have been the best choice for many years. However, in more recent times, the falling film shell and tube type has entered the market. In both the flooded and falling film types of heat exchanger, a serpentine conductive pipe runs through a surrounding shell. The refrigerant is passed through the shell while water is passed through the interior of the conductive pipe. The serpentine pipe is typically made of copper. Heat transfer is increased by creating a rough surface on the pipe's exterior (such as by knurling, peening, etc.) and rifling the pipe's interior to create rotational turbulent flow in the water.
In the flooded type the copper tubes are completely submerged in a tank of boiling liquid refrigerant and the heat transfer is enhanced over what would be achieved using plain copper pipe by increasing the outer surface area of the copper pipe using specially designed knurling. Although the flooded type produces very efficient heat transfer, it does have drawbacks. The main drawback is the excessive amount of liquid refrigerant required to completely submerge the copper tubes (an excessive “gas charge”). The HVAC industry is now under pressure to reduce the gas charge in systems owing to expense and the potential for environmental contamination.
The “falling film” approach differs significantly from the flooded tube exchanger. It still employs a shell enclosing a serpentine path of conductive piping (which contains the secondary fluid). However, instead of flooding the shell with boiling refrigerant, the refrigerant is sprayed or otherwise deposited onto the outer surface of the piping. This produces a thin film of refrigerant, which rapidly evaporates. This film cascades downward under the influence of gravity. Hence the name “falling film.” The surfaces of the heat exchanger must typically be carefully designed in order to completely cover the appropriate surfaces with the falling film and to properly direct its downward flow. Those skilled in the art will appreciate the fact that the falling film approach—if properly designed—can use substantially less refrigerant.
Both the flooded and falling film types are shell and tube heat exchangers. Vaporized refrigerant is drawn out of the top of the shell, while liquid refrigerant tends to collect in the bottom (either as a pool of boiling liquid in the case of the flooded type or a cascade of film flow in the falling film type). The tendency of the liquid refrigerant to collect in the bottom of the shell means that the oil circulating in the system also tends to become trapped there. This can cause continual oil shortage problems at the compressor unless special oil recovery devices are added to the design of the heat exchanger. The entrapped oil decreases the efficiency of the heat exchanger and—of course—potentially starves the compressor of lubrication. One of the main causes of compressor failure in refrigeration systems is inadequate oil return. Significant engineering effort goes into avoiding oil starvation.
An oil free compressor has been developed. This device is described in detail in U.S. Pat. No. 5,857,348 to Conry. This development does eliminate the oil accumulation problem associated with flooded shell-and-tube heat exchangers. However, as discussed earlier, another known shortcoming of such heat exchangers is the requirement of a relatively large mass of circulating refrigerant. A large mass of refrigerant is required (a large “gas charge”) to fully cover all the tubes in a shell-and-tube type exchanger. Such heat exchangers are not very space efficient. Most include a large volume of open space within the shell surrounding a small volume within the tubes. Thus, in order to completely cover the tubes with refrigerant, a relatively large mass of refrigerant is required.
Global warming and other environmental concerns disfavor the use of large gas charges. An HVAC system having a minimal gas charge is preferable. One way of minimizing the required gas charge is the use of a modified plate type heat exchanger incorporating the falling film approach. This type of exchanger can be much more space efficient. The present invention proposes just such a heat exchanger, in which the heat transfer across the plates is greatly increased by modifying the surface texture of the primary fluid side of each plate and applying a thin film of refrigerant to the modified surface.